To prepare sauce, cook onions until tender in a large saucepan. Add 29 ounces tomatoes, water, and spaghetti sauce mix. Bring to a boil. Reduce heat and simmer for ten minutes, or until heated through. Meanwhile, cook cook chicken until no longer pink. Transfer to a 3 1/2-quart cooker. Stir in marinara sauce, remaining can of tomatoes, basil, oregano, and garlic powder. Mix well. Cover and cook on low heat for seven hours. Stir in tortellini and sprinkle with cheese. Cover and cook on low heat again for 15 minutes, or until tortellini is cooked.